1. Field of the Invention
The present invention concerns a gradient coil of a magnetic resonance apparatus.
2. Description of the Prior Art
A magnetic resonance apparatus has a gradient coil system that contains three (sub-)gradient coils. For example, a magnetic field gradient is generated in the X-direction with the use of a first gradient coil while a magnetic field gradient in the Y-direction is generated with the use of a second gradient coil, and a third gradient coil generates a magnetic field gradient in the Z-direction.
The XY gradient coils are known as “saddle coils” due to the shape of their design.
It is known to design saddle coils composed of bundled individual wires. In such a coil design, conductor loops that typically include one to six bundled individual wires are fixed on a carrier (substrate) plate, for example by gluing.
The individual wires of a conductor loop are each provided with a lacquer insulation layer (typically 2 to 10 μm in thickness) and are therefore insulated from one another. A common current signal flows through the individual wires of the conductor loops in order to form the gradient field (magnetic field) in an examination region of the magnetic resonance system.
Such saddle coils enable the realization of an optimized current density in a predetermined central coil region in order to form the desired magnetic field in the examination region.
Coil windings made of individual wires offer the advantage that a high number of coil windings can be arranged in a predetermined central coil region.
A disadvantage of coils of this type is a relatively high ohmic resistance due to the individual wires that are used.
It is also known to fashion saddle coils using an electrically conductive plate. For example, elliptically running divider structures (known as traces) are milled into the electrically conductive plate. The plate is subsequently formed into the saddle shape—for example by bending the plate in the shape of a half-cylinder shell.
The traces can also be generated by cutting techniques (for example water jet cutting, laser cutting, etc.) or with the use of punching techniques.
By the curving of the plate into a saddle shape and by the effect of the separating traces, conductor structures are formed that, charged with a current signal, form a desired X-gradient field or Y-gradient field.
The magnetic field efficiency is determined by the maximum achievable current density in a central region of the plate. The requirements for a minimal insulation distance between the coil windings or conductor structures in this middle region result from this factor. Maximum conductor structure cross-sections are used for this determination.
The power consumption of a gradient system from the mains is determined by the ohmic resistance of the gradient coils. A necessity to maximize the cross-section of the respective conductor structures results from this consideration in order to be able to utilize only a limited mains power available to the customer.
Given currents of 500 A to 1000 A that are typical today, in general it is necessary to use 20 to 30 individual conductor loops per quadrant on the plate of the latter type of saddle coil.
The advantage of a gradient coil created from an electrically conductive plate is that the gradient coil has a very low ohmic resistance because a large-area conductive surface, that is reduced only by the width of the trace, is available. Depending on the technique, this trace can be very narrow—even only a few millimeters wide.
A disadvantage, however, is that a dense conductor trace population in the central coil regions can be achieved only with difficulty, because a predetermined winding distance must be maintained in order to maintain insulation stability.